Internet Draft J. Chu
draft-ietf-tcpm-initcwnd-02.txt N. Dukkipati
Intended status: Standard Y. Cheng
Updates: 3390, 5681 M. Mathis
Creation date: October 16, 2011 Google, Inc.
Expiration date: April 2012
Increasing TCP's Initial Window
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This document updates RFC 3390 to raise the upper bound on TCP's
initial window (IW) to 10 segments or roughly 15KB. It is patterned
after and borrows heavily from RFC 3390 [RFC3390] and earlier work in
this area.
The primary argument in favor of raising IW follows from the evolving
scale of the Internet. Ten segments are likely to fit into queue
space available at any broadband access link, even when there are a
reasonable number of concurrent connections.
Lower speed links can be treated with environment specific
configurations, such that they can be protected from being
overwhelmed by large initial window bursts without imposing a
suboptimal initial window on the rest of the Internet.
This document reviews the advantages and disadvantages of using a
larger initial window, and includes summaries of several large scale
experiments showing that an initial window of 10 segments provides
benefits across the board for a variety of BW, RTT, and BDP classes.
These results show significant benefits for increasing IW for users
at much smaller data rates than had been previously anticipated.
However, at initial windows larger than 10, the results are mixed. We
believe that these mixed results are not intrinsic, but are the
consequence of various implementation artifacts, including overly
aggressive applications employing many simultaneous connections.
We propose that all TCP implementations should have a settable TCP IW
parameter; the default setting may start at 10 segments and should be
raised as we come to understand and and correct things that conflict.
In addition, we introduce a minor revision to RFC 3390 and RFC 5681
[RFC5681] to eliminate resetting the initial window when the SYN or
SYN/ACK is lost.
The document closes with a discussion of a list of concerns that have
been brought up, and some recent test results showing most of the
concerns can not be validated.
A complementary set of slides for this proposal can be found at
[CD10].
2. TCP Modification
This document proposes an increase in the permitted upper bound for
TCP's initial window (IW) to 10 segments. This increase is optional:
a TCP MAY start with a larger initial window up to 10 segments.
This upper bound for the initial window size represents a change from
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Internet Draft Increasing TCP's Initial Window October 2011RFC 3390 [RFC3390], which specified that the congestion window be
initialized between 2 and 4 segments depending on the MSS.
This change applies to the initial window of the connection in the
first round trip time (RTT) of data transmission following the TCP
three-way handshake. Neither the SYN/ACK nor its acknowledgment (ACK)
in the three-way handshake should increase the initial window size.
Furthermore, RFC 3390 and RFC 5681 [RFC5681] state that
"If the SYN or SYN/ACK is lost, the initial window used by a
sender after a correctly transmitted SYN MUST be one segment
consisting of MSS bytes."
The proposed change to reduce the default RTO to 1 second [RFC6298]
increases the chance for spurious SYN or SYN/ACK retransmission, thus
unnecessarily penalizing connections with RTT > 1 second if their
initial window is reduced to 1 segment. For this reason, it is
RECOMMENDED that implementations refrain from resetting the initial
window to 1 segment, unless either there have been multiple SYN or
SYN/ACK retransmissions, or true loss detection has been made.
TCP implementations use slow start in as many as three different
ways: (1) to start a new connection (the initial window); (2) to
restart transmission after a long idle period (the restart window);
and (3) to restart transmission after a retransmit timeout (the loss
window). The change specified in this document affects the value of
the initial window. Optionally, a TCP MAY set the restart window to
the minimum of the value used for the initial window and the current
value of cwnd (in other words, using a larger value for the restart
window should never increase the size of cwnd). These changes do NOT
change the loss window, which must remain 1 segment of MSS bytes (to
permit the lowest possible window size in the case of severe
congestion).
Furthermore, to limit any negative effect that a larger initial
window may have on links with limited bandwidth or buffer space,
implementations SHOULD fall back to RFC 3390 for the restart window
(RW), if any packet loss is detected during either the initial
window, or a restart window, when more than 4KB of data is sent.
3. Implementation Issues
[Need to decide if a different formula is needed for PMTU != 1500.]
HTTP 1.1 specification allows only two simultaneous connections per
domain, while web browsers open more simultaneous TCP connections
[Ste08], partly to circumvent the small initial window in order to
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speed up the loading of web pages as described above.
When web browsers open simultaneous TCP connections to the same
destination, they are working against TCP's congestion control
mechanisms [FF99]. Combining this behavior with larger initial
windows further increases the burstiness and unfairness to other
traffic in the network. A larger initial window will incentivize
applications to use fewer concurrent TCP connections.
Some implementations advertise small initial receive window (Table 2
in [Duk10]), effectively limiting how much window a remote host may
use. In order to realize the full benefit of the large initial
window, implementations are encouraged to advertise an initial
receive window of at least 10 segments, except for the circumstances
where a larger initial window is deemed harmful. (See the Mitigation
section below.)
TCP SACK option ([RFC2018]) was thought to be required in order for
the larger initial window to perform well. But measurements from both
a testbed and live tests showed that IW=10 without the SACK option
still beats the performance of IW=3 with the SACK option [CW10].
4. Background
TCP congestion window was introduced as part of the congestion
control algorithm by Van Jacobson in 1988 [Jac88]. The initial value
of one segment was used as the starting point for newly established
connections to probe the available bandwidth on the network.
Today's Internet is dominated by web traffic running on top of short-
lived TCP connections [IOR2009]. The relatively small initial window
has become a limiting factor for the performance of many web
applications.
The global Internet has continued to grow, both in speed and
penetration. According to the latest report from Akamai [AKAM10], the
global broadband (> 2Mbps) adoption has surpassed 50%, propelling the
average connection speed to reach 1.7Mbps, while the narrowband (<
256Kbps) usage has dropped to 5%. In contrast, TCP's initial window
has remained 4KB for a decade [RFC2414], corresponding to a bandwidth
utilization of less than 200Kbps per connection, assuming an RTT of
200ms.
A large proportion of flows on the Internet are short web
transactions over TCP, and complete before exiting TCP slow start.
Speeding up the TCP flow startup phase, including circumventing the
initial window limit, has been an area of active research [PWSB09,
Sch08]. Numerous proposals exist [LAJW07, RFC4782, PRAKS02, PK98].
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Some require router support [RFC4782, PK98], hence are not practical
for the public Internet. Others suggested bold, but often radical
ideas, likely requiring more years of research before standardization
and deployment.
In the mean time, applications have responded to TCP's "slow" start.
Web sites use multiple sub-domains [Bel10] to circumvent HTTP 1.1
regulation on two connections per physical host [RFC2616]. As of
today, major web browsers open multiple connections to the same site
(up to six connections per domain [Ste08] and the number is growing).
This trend is to remedy HTTP serialized download to achieve
parallelism and higher performance. But it also implies today most
access links are severely under-utilized, hence having multiple TCP
connections improves performance most of the time. While raising the
initial congestion window may cause congestion for certain users
using these browsers, we argue that the browsers and other
application need to respect HTTP 1.1 regulation and stop increasing
number of simultaneous TCP connections. We believe a modest increase
of the initial window will help to stop this trend, and provide the
best interim solution to improve overall user performance, and reduce
the server, client, and network load.
Note that persistent connections and pipelining are designed to
address some of the issues with HTTP above [RFC2616]. Their presence
does not diminish the need for a larger initial window. E.g., data
from the Chrome browser show that 35% of HTTP requests are made on
new TCP connections. Our test data also confirm significant latency
reduction with the large initial window even with these two HTTP
features ([Duk10]).
Also note that packet pacing has been suggested as an effective
mechanism to avoid large bursts and their associated damage [VH97].
We do not require pacing in our proposal due to our strong preference
for a simple solution. We suspect for packet bursts of a moderate
size, packet pacing will not be necessary. This seems to be confirmed
by our test results.
More discussion of the increase in initial window, including the
choice of 10 segments can be found in [Duk10, CD10].
5. Advantages of Larger Initial Windows5.1 Reducing Latency
An increase of the initial window from 3 segments to 10 segments
reduces the total transfer time for data sets greater than 4KB by up
to 4 round trips.
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The table below compares the number of round trips between IW=3 and
IW=10 for different transfer sizes, assuming infinite bandwidth, no
packet loss, and the standard delayed acks with large delayed-ack
timer.
---------------------------------------
| total segments | IW=3 | IW=10 |
---------------------------------------
| 3 | 1 | 1 |
| 6 | 2 | 1 |
| 10 | 3 | 1 |
| 12 | 3 | 2 |
| 21 | 4 | 2 |
| 25 | 5 | 2 |
| 33 | 5 | 3 |
| 46 | 6 | 3 |
| 51 | 6 | 4 |
| 78 | 7 | 4 |
| 79 | 8 | 4 |
| 120 | 8 | 5 |
| 127 | 9 | 5 |
---------------------------------------
For example, with the larger initial window, a transfer of 32
segments of data will require only two rather than five round trips
to complete.
5.2 Keeping up with the growth of web object sizeRFC 3390 stated that the main motivation for increasing the initial
window to 4KB was to speed up connections that only transmit a small
amount of data, e.g., email and web. The majority of transfers back
then were less than 4KB, and could be completed in a single RTT
[All00].
Since RFC 3390 was published, web objects have gotten significantly
larger [Chu09, RJ10]. Today only a small percentage of web objects
(e.g., 10% of Google's search responses) can fit in the 4KB initial
window. The average HTTP response size of gmail.com, a highly
scripted web-site, is 8KB (Figure 1. in [Duk10]). The average web
page, including all static and dynamic scripted web objects on the
page, has seen even greater growth in size [RJ10]. HTTP pipelining
[RFC2616] and new web transport protocols like SPDY [SPDY] allow
multiple web objects to be sent in a single transaction, potentially
requiring even larger initial window in order to transfer a whole web
page in one round trip.
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A greater-than-3-segment initial window increases the chance to
recover packet loss through Fast Retransmit rather than the lengthy
initial RTO [RFC5681]. This is because the fast retransmit algorithm
requires three duplicate acks as an indication that a segment has
been lost rather than reordered. While newer loss recovery techniques
such as Limited Transmit [RFC3042] and Early Retransmit [RFC5827]
have been proposed to help speeding up loss recovery from a smaller
window, both algorithms can still benefit from the larger initial
window because of a better chance to receive more ACKs to react upon.
6. Disadvantages of Larger Initial Windows for the Individual Connection
The larger bursts from an increase in the initial window may cause
buffer overrun and packet drop in routers with small buffers, or
routers experiencing congestion. This could result in unnecessary
retransmit timeouts. For a large-window connection that is able to
recover without a retransmit timeout, this could result in an
unnecessarily-early transition from the slow-start to the congestion-
avoidance phase of the window increase algorithm. [Note: knowing the
large initial window may cause premature segment drop, should one
make an exception for it, i.e., by allowing ssthresh to remain
unchanged if loss is from an enlarged initial window?]
Premature segment drops are unlikely to occur in uncongested networks
with sufficient buffering, or in moderately-congested networks where
the congested router uses active queue management (such as Random
Early Detection [FJ93, RFC2309, RFC3150]).
Insufficient buffering is more likely to exist in the access routers
connecting slower links. A recent study of access router buffer size
[DGHS07] reveals the majority of access routers provision enough
buffer for 130ms or longer, sufficient to cover a burst of more than
10 packets at 1Mbps speed, but possibly not sufficient for browsers
opening simultaneous connections.
A testbed study [CW10] on the effect of the larger initial window
with five simultaneously opened connections revealed that, even with
limited buffer size on slow links, IW=10 still reduced the total
latency of web transactions, although at the cost of higher packet
drop rates as compared to IW=3.
Some TCP connections will receive better performance with the larger
initial window even if the burstiness of the initial window results
in premature segment drops. This will be true if (1) the TCP
connection recovers from the segment drop without a retransmit
timeout, and (2) the TCP connection is ultimately limited to a small
congestion window by either network congestion or by the receiver's
advertised window.
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Internet Draft Increasing TCP's Initial Window October 20117. Disadvantages of Larger Initial Windows for the Network
An increase in the initial window may increase congestion in a
network. However, since the increase is one-time only (at the
beginning of a connection), and the rest of TCP's congestion backoff
mechanism remains in place, it's highly unlikely the increase will
render a network in a persistent state of congestion, or even
congestion collapse. This seems to have been confirmed by our large
scale experiments described later.
Some of the discussions from RFC 3390 are still valid for IW=10.
Moreover, it is worth noting that although TCP NewReno increases the
chance of duplicate segments when trying to recover multiple packet
losses from a large window [RFC3782], the wide support of TCP
Selective Acknowledgment (SACK) option [RFC2018] in all major OSes
today should keep the volume of duplicate segments in check.
Recent measurements [Get11] provide evidence of extremely large
queues (in the order of one second) at access networks of the
Internet. While a significant part of the buffer bloat is contributed
by large downloads/uploads such as video files, emails with large
attachments, backups and download of movies to disk, some of the
problem is also caused by Web browsing of image heavy sites [Get11].
This queuing delay is generally considered harmful for responsiveness
of latency sensitive traffic such as DNS queries, ARP, DHCP, VoIP and
Gaming. IW=10 can exacerbate this problem when doing short downloads
such as Web browsing. The mitigations proposed for the broader
problem of buffer bloating are also applicable in this case, such as
the use of ECN, AQM schemes and traffic classification (QoS).
8. Mitigation of Negative Impact
Much of the negative impact from an increase in the initial window is
likely to be felt by users behind slow links with limited buffers.
The negative impact can be mitigated by hosts directly connected to a
low-speed link advertising a smaller initial receive window than 10
segments. This can be achieved either through manual configuration by
the users, or through the host stack auto-detecting the low bandwidth
links.
More suggestions to improve the end-to-end performance of slow links
can be found in RFC 3150 [RFC3150].
[Note: if packet loss is detected during IW through fast retransmit,
should cwnd back down to 2 rather than FlightSize / 2?]
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A large initial window increases the chance of spurious RTO on a low-
bandwidth path because the packet transmission time will dominate the
round-trip time. To minimize spurious retransmissions,
implementations MUST follow RFC 2988 [RFC2988] to restart the
retransmission timer with the current value of RTO for each ack
received that acknowledges new data.
10. Experimental Results From Large Scale Cluster Tests
In this section we summarize our findings from large scale Internet
experiments with an initial window of 10 segments, conducted via
Google's front-end infrastructure serving a diverse set of
applications. We present results from two data centers, each chosen
because of the specific characteristics of subnets served: AvgDC has
connection bandwidths closer to the worldwide average reported in
[AKAM10], with a median connection speed of about 1.7Mbps; SlowDC has
a larger proportion of traffic from slow bandwidth subnets with
nearly 20% of traffic from connections below 100Kbps, and a third
below 256Kbps.
Guided by measurements data, we answer two key questions: what is the
latency benefit when TCP connections start with a higher initial
window, and on the flip side, what is the cost?
10.1 The benefits
The average web search latency improvement over all responses in
AvgDC is 11.7% (68 ms) and 8.7% (72 ms) in SlowDC. We further
analyzed the data based on traffic characteristics and subnet
properties such as bandwidth (BW), round-trip time (RTT), and
bandwidth-delay product (BDP). The average response latency improved
across the board for a variety of subnets with the largest benefits
of over 20% from high RTT and high BDP networks, wherein most
responses can fit within the pipe. Correspondingly, responses from
low RTT paths experienced the smallest improvements of about 5%.
Contrary to what we expected, responses from low bandwidth subnets
experienced the best latency improvements (between 10-20%) in the
buckets 0-56Kbps and 56-256Kbps buckets. We speculate low BW networks
observe improved latency for two plausible reasons: 1) fewer slow-
start rounds: unlike many large BW networks, low BW subnets with
dial-up modems have inherently large RTTs; and 2) faster loss
recovery: an initial window larger than 3 segments increases the
chances of a lost packet to be recovered through Fast Retransmit as
opposed to a lengthy RTO.
Responses of different sizes benefited to varying degrees; those
larger than 3 segments naturally demonstrated larger improvements,
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because they finished in fewer rounds in slow start as compared to
the baseline. In our experiments, response sizes <= 3 segments also
demonstrated small latency benefits.
To find out how individual subnets performed, we analyzed average
latency at a /24 subnet level (an approximation to a user base
offered similar set of services by a common ISP). We find even at the
subnet granularity, latency improved at all quantiles ranging from 5-
11%.
10.2 The cost
To quantify the cost of raising the initial window, we analyzed the
data specifically for subnets with low bandwidth and BDP,
retransmission rates for different kinds of applications, as well as
latency for applications operating with multiple concurrent TCP
connections. From our measurements we found no evidence of a negative
latency impacts that correlate to BW or BDP alone, but in fact both
kinds of subnets demonstrated latency improvements across averages
and quantiles.
As expected, the retransmission rate increased modestly when
operating with larger initial congestion window. The overall increase
in AvgDC is 0.3% (from 1.98% to 2.29%) and in SlowDC is 0.7% (from
3.54% to 4.21%). In our investigation, with the exception of one
application, the larger window resulted in a retransmission increase
of < 0.5% for services in the AvgDC. The exception is the Maps
application that operates with multiple concurrent TCP connections,
which increased its retransmission rate by 0.9% in AvgDC and 1.85% in
SlowDC (from 3.94% to 5.79%).
In our experiments, the percentage of traffic experiencing
retransmissions did not increase significantly. E.g. 90% of web
search and maps experienced zero retransmission in SlowDC
(percentages are higher for AvgDC); a break up of retransmissions by
percentiles indicate that most increases come from portion of traffic
already experiencing retransmissions in the baseline with initial
window of 3 segments.
Traffic patterns from applications using multiple concurrent TCP
connections all operating with a large initial window represent one
of the worst case scenarios where latency can be adversely impacted
due to bottleneck buffer overflow. Our investigation shows that such
a traffic pattern has not been a problem in AvgDC, where all these
applications, specifically maps and image thumbnails, demonstrated
improved latencies varying from 2-20%. In the case of SlowDC, while
these applications continued showing a latency improvement in the
mean, their latencies in higher quantiles (96 and above for maps)
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indicated instances where latency with larger window is worse than
the baseline, e.g. the 99% latency for maps has increased by 2.3%
(80ms) when compared to the baseline. There is no evidence from our
measurements that such a cost on latency is a result of subnet
bandwidth alone. Although we have no way of knowing from our data, we
conjecture that the amount of buffering at bottleneck links plays a
key role in performance of these applications.
Further details on our experiments and analysis can be found in
[Duk10, DCCM10].
11. List of Concerns and Corresponding Test Results
Concerns have been raised since we first published our proposal based
on a set of large scale experiments. To better understand the impact
of a larger initial window in order to confirm or dismiss these
concerns, we, as well as people outside of Google have conducted
numerous additional tests in the past year, using either Google's
large scale clusters, simulations, or real testbeds. The following is
a list of concerns and some of the findings.
A complete list of tests conducted, their results and related studies
can be found at [IW10].
o How complete are our tests in traffic pattern coverage?
Google today offers a large portfolio of services beyond web
search. The list includes Gmail, Google Maps, Photos, News, Sites,
Images, Videos,..., etc. Our tests included most of Google's
services, covering a wide variety of traffic sizes and patterns.
One notable exception is YouTube because we don't think the large
initial window will have much material impact, either positive or
negative, on bulk data services.
[CW10] contains some result from a testbed study on how short flows
with a larger initial window might affect the throughput
performance of other co-existing, long lived, bulk data transfers.
o Larger bursts from the increase in the initial window cause
significantly more packet drops
All the known tests conducted on this subject so far [Duk10, Sch11,
Sch11-1, CW10] show that, although bursts from the larger initial
window tend to cause more packet drops, the increase tends to be
very modest. The only exception is from our own testbed study
[CW10] when under extremely high load and/or simultaneous opens.
But both IW=3 and IW=10 suffered very high packet loss rates under
those conditions.
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o A large initial window may severely impact TCP performance over
highly multiplexed links still common in developing regions
Our large scale experiments described in section 10 above also
covered Africa and South America. Measurement data from those
regions [DCCM10] revealed improved latency even for those Google
services that employ multiple simultaneous connections, at the cost
of small increase in the retransmission rate. It seems that the
round trip savings from a larger initial window more than make up
the time spent on recovering more lost packets.
Similar phenomenon have also been observed from our testbed study
[CW10].
o Why 10 segments?
Questions have been raised on how the number 10 was picked. We have
tried different sizes in our large scale experiments, and found
that 10 segments seem to give most of the benefits for the services
we tested while not causing significant increase in the
retransmission rates. Going forward 10 segments may turn out to be
too small when the average of web object sizes continue to grow. A
scheme to attempt to right size the initial window automatically
over long timescales has been proposed in [Tou10].
o Need more thorough analysis of the impact on slow links
Although data from [Duk10] showed the large initial window reduced
the average latency even for the dialup link class of only 56Kbps
in bandwidth, it is only prudent to perform more microscopic
analysis on its effect on slow links. We set up two testbeds for
this purpose [CW10].
Both testbeds were used to emulate a 300ms RTT, bottleneck link
bandwidth as low as 64Kbps, and route queue size as low as 40
packets. Although we've tried a large combination of test
parameters, almost all tests we ran managed to show some latency
improvement from IW=10, with only a modest increase in the packet
drop rate until a very high load was injected. The testbed result
was consistent with both our own large scale data center
experiments [CD10, DCCM10] and a separate study using NSC
simulations [Sch11, Sch11-1].
o How will the larger initial window affect flows with initial
windows 4KB or less?
Flows with the larger initial window will likely grab more
bandwidth from a bottleneck link when competing against flows with
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smaller initial window, at least initially. How long will this
"unfairness" last? Will there be any "capture effect" where flows
with larger initial window possess a disproportional share of
bandwidth beyond just a few round trips?
If there is any "unfairness" issue from flows with different
initial windows, it did not show up in our large scale experiments,
as the average latency for the bucket of all responses < 4KB did
not seem to be affected by the presence of many other larger
responses employing large initial window. As a matter of fact they
seemed to benefit from the large initial window too, as shown in
Figure 7 of [Duk10].
The same phenomenon seems to exist in our testbed experiments.
Flows with IW=3 only suffered slightly when competing against flows
with IW=10 in light to median loads. Under high load both flows'
latency improved when mixed together. Also long-lived, background
bulk-data flows seemed to enjoy higher throughput when running
against many foreground short flows of IW=10 than against short
flows of IW=3. One plausible explanation was IW=10 enabled short
flows to complete sooner, leaving more room for the long-lived,
background flows.
An independent study using NSC simulator has also concluded that
IW=10 works rather well and is quite fair against IW=3 [Sch11,
Sch11-1].
o How will a larger initial window perform over cellular networks?
Some simulation studies [JNDK10, JNDK10-1] have been conducted to
study the effect of a larger initial window on wireless links from
2G to 4G networks (EGDE/HSPA/LTE). The overall result seems mixed
in both raw performance and the fairness index.
There has been on-going studies by people from Nokia on the effect
of a larger initial window on GPRS and HSDPA networks. Initial test
results seem to show no or little improvement from flows with a
larger initial window. More studies are needed to understand why.
12. Related Proposals
Two other proposals [All10, Tou10] have been made with the goal to
raise TCP's initial window size over a large timescale. Both aim at
addressing the concern about the uncertain impact from raising the
initial window size at an Internet wide scale. Moreover, [Tou10]
seeks an algorithm to automate the adjustment of IW safely over long
haul period.
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Based on our test results from the past couple of years, we believe
our proposal - a modest, static increase of IW to 10, to be the best
near-term solution that is both simple and effective. The other
proposals, with their added complexity and much longer deployment
cycles, seem best suited for growing IW beyond 10 in the long run.
13. Security Considerations
This document discusses the initial congestion window permitted for
TCP connections. Changing this value does not raise any known new
security issues with TCP.
14. Conclusion
This document suggests a simple change to TCP that will reduce the
application latency over short-lived TCP connections or links with
long RTTs (saving several RTTs during the initial slow-start phase)
with little or no negative impact over other flows. Extensive tests
have been conducted through both testbeds and large data centers with
most results showing improved latency with only a small increase in
the packet retransmission rate. Based on these results we believe a
modest increase of IW to 10 is the best near-term proposal while
other proposals [All10, Tou10] may be best suited to grow IW beyond
10 in the long run.
15. IANA Considerations
None
16. Acknowledgments
Many people at Google have helped to make the set of large scale
tests possible. We would especially like to acknowledge Amit Agarwal,
Tom Herbert, Arvind Jain and Tiziana Refice for their major
contributions.
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